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2011. május 20., péntek

Definitions of life by Carl Sagan essay

Definitions of lifeby Carl Sagan

A great deal is known about life. Anatomists and taxonomists have studied the forms and

relations of more than a million separate species of plants and animals. Physiologists have

investigated the gross functioning of organisms. Biochemists have probed the biological

interactions of the organic molecules that make up life on our planet. Molecular biologists

have uncovered the very molecules responsible for reproduction and for the passage of

hereditary information from generation to generation, a subject that geneticists had

previously studied without going to the molecular level. Ecologists have inquired into the

relations between organisms and their environments, ethologists the behavior of animals

and plants, embryologists the development of complex organisms from a single cell,

evolutionary biologists the emergence of organisms from pre-existing forms over geological

time. Yet despite the enormous fund of information that each of these biological specialties

has provided, it is a remarkable fact that no general agreement exists on what it is that is

being studied. There is no generally accepted definition of life. In fact, there is a certain clearly

discernible tendency for each biological specialty to define life in its own terms. The average

person also tends to think of life in his own terms. For example, the man in the street, if asked

about life on other planets, will often picture life of a distinctly human sort. Many individuals

believe that insects are not animals, because by "animals" they mean "mammals." Man tends

to define in terms of the familiar. But the fundamental truths may not be familiar. Of the

following definitions, the first two are in terms familiar in everyday life; the next three are

based on more abstract concepts and theoretical frameworks.

Physiological

For many years a physiological definition of life was popular. Life was defined as any system

capable of performing a number of such functions as eating, metabolizing, excreting,

breathing, moving, growing, reproducing, and being responsive to external stimuli. But many

such properties are either present in machines that nobody is willing to call alive, or absent

from organisms that everybody is willing to call alive. An automobile, for example, can be said

to eat, metabolize, excrete, breathe, move, and be responsive to external stimuli. And a visitor

from another planet, judging from the enormous numbers of automobiles on the Earth and

the way in which cities and landscapes have been designed for the special benefit of

motorcars, might well believe that automobiles are not only alive but are the dominant life

form on the planet. Man, however, professes to know better. On the other hand, some

bacteria do not breathe at all but instead live out their days by altering the oxidation state of

sulfur.

Metabolic

The metabolic definition is still popular with many biologists. It describes a living system as an

object with a definite boundary, continually exchanging some of its materials with its

surroundings, but without altering its general properties, at least over some period of time.

But again there are exceptions. There are seeds and spores that remain, so far as is known,

perfectly dormant and totally without metabolic activity at low temperatures for hundreds,

perhaps thousands, of years but that can revive perfectly well upon being subjected to more

clement conditions. A flame, such as that of a candle in a closed room, will have a perfectly

defined shape with fixed boundary and will be maintained by the combination of its organic

waxes with molecular oxygen, producing carbon dioxide and water. A similar chemical

reaction, incidentally, is fundamental to most animal life on Earth. Flames also have a wellknown

capacity for growth.

Biochemical

A biochemical or molecular biological definition sees living organisms as systems that contain

reproducible hereditary information coded in nucleic acid molecules and that metabolize by

controlling the rate of chemical reactions using proteinaceous catalysts known as enzymes. In

many respects, this is more satisfying than the physiological or metabolic definitions of life.

There are, however, even here, the hints of counterexamples. There seems to be some

evidence that a virus-like agent called scrapie contains no nucleic acids at all, although it

has been hypothesized that the nucleic acids of the host animal may nevertheless be involved

in the reproduction of scrapie. Furthermore, a definition strictly in chemical terms seems

peculiarly vulnerable. It implies that, were a person able to construct a system that had all the

functional properties of life, it would still not be alive if it lacked the molecules that earthly

biologists are fond of--and made of.

Genetic

All organisms on Earth, from the simplest cell to man himself, are machines of extraordinary

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